1. Field of the Invention
This invention relates to a contrast adjustment circuit for a liquid crystal display (LCD).
2. Description of the Related Art
An LCD is a device generally constructed such that a pair of transparent conductive films (electrodes) between which liquid crystal is enclosed are held between a pair of glass plates and a driving voltage is applied to the transparent conductive films to change the arrangement or the phase of molecules of the liquid crystal by an action of an electric field to change the amount of light passing through the liquid crystal to make a display. Such LCDs are utilized frequently as display devices for computers or like apparatus, which will be hereinafter referred to as electronic apparatus, and the adjustment of the contrast (degree of the difference in brightness between a brightest portion and a darkest portion of a screen) of an LCD is performed by finely adjusting a variable resistor of a contrast adjustment circuit from an electronic apparatus. The present invention is directed to such a contrast adjustment circuit for an LCD as described just above.
FIG. 1 is a circuit diagram showing a prior art. Referring to FIG. 1, reference numeral 1 denotes an electronic apparatus such as a computer, and 2 a liquid crystal display having an LCD panel 3. The electronic apparatus 1 includes a first power source (V1) 4 and a second power source (V2) 5 which are connected to each other by way of a plurality of voltage dividing resistors 6, 6 and 7. The resistor 7 is formed as a variable resistor, and an output terminal at an end of the variable resistor 7 is connected to a connection terminal 8 to the liquid crystal display module 2.
The liquid crystal display module 2 includes a transistor 9. The base terminal of the transistor 9 is connected to the connection terminal 8; the emitter terminal is connected to the first power source 4 supplied from the electronic apparatus 1 by way of a plurality of voltage dividing resistors 10; and the collector terminal is connected to the second power source 5 supplied from the electronic apparatus 1. The emitter terminal of the transistor 9 is connected to a corresponding one of a plurality of driving voltage input terminals of a control IC (not shown) of the LCD panel 3, and also output terminals at one ends of the individual voltage dividing resistors 10 are connected to corresponding ones of the driving voltage input terminals of control IC. Driving voltages V.sub.LCD(1), . . . , V.sub.LCD(n-1), V.sub.LCD(n) are selectively supplied to the electrodes (transparent conductive films) of the LCD panel 3 under the control of the control IC.
The variable resistor 7 of the electronic apparatus 1 is adjusted finely to adjust the driving voltage V.sub.LCD (for example, -20 V) to the LCD panel 3 within a predetermined range (for example, the range of .+-.2 V) in order to adjust the contrast of the LCD panel 3. It is to be noted that a contrast adjustment circuit is constituted from the first power source 4, the second power source 5, the voltage dividing resistors 6, 6 and 7 including the variable resistor 7, and the transistor 9.
FIG. 2 is a diagram illustrating the relationship between the contrast and the driving voltage (V.sub.LCD) of an LCD. By finely adjusting the variable resistor 7, the contrast is adjusted within a contrast range (indicated by "a" in FIG. 2) and normally set within an optimum contrast range indicated by b in FIG. 2 when the LCD is used. It is to be noted that the point c or d in FIG. 2 indicates a point at which the white or black display becomes a same color as the color of the background of the LCD panel, and in FIG. 2, the driving voltage V.sub.LCD is -18 V and -22 V at the points c and d, respectively. Meanwhile, the point e indicates a point at which the difference in brightness exhibits a maximum value (the driving voltage then is called contrast peak voltage).
By the way, in recent years, there is a tendency that LCD panels provided from different makers are connected, when to be used, compatibly to an electronic apparatus, and since LCD panels have different optimum contrast voltages depending upon the material of liquid crystal employed or some other factor, a conventional contrast adjustment circuit cannot perform adjustment of the contrast within an optimum contrast range, or else, the construction of the contrast adjustment circuit must be modified so as to conform to the contrast peak voltage of an LCD panel to be employed. Accordingly, the conventional contrast adjustment circuit is disadvantageous in that it lacks in flexibility.